In the drilling of boreholes such as oil or gas wells, a drilling fluid (hereinafter referred to as “drilling mud”) is circulated through the well during drilling in order to, inter alia, remove drilled cuttings, balance the pressure of formation fluids to prevent influxes and maintain the stability of the borehole. In order to provide the required density and viscosity, the mud can include certain solids such as barite and bentonite as well as solids derived from the drilling action. When drilling oil and gas wells, it is quite common to encounter subterranean formations which are porous. If the hydrostatic pressure of the drilling fluid is greater than the pressure of fluids in such formations, mud will penetrate the formation. Generally the pore size of such formations is sufficient to admit the liquid components and very fine solids but to filter out the other solids such as barite or bentonite. These filtered solids form on the borehole wall as a filtercake. This filtercake grows and compacts until the differential pressure is balanced by the stress in the filtercake.
The filtercake is typically many times more concentrated than the drilling fluid, and has a shear strength many orders of magnitude greater. It also has a much lower permeability than the formation. Filtercake can therefore be useful as it acts as a barrier to prevent fluid loss from the drilling mud to the formation.
However if the filtercake becomes too thick, problems can occur. For example, filtercake is a significant contributor to differential sticking: if the drill string or bottomhole assembly (BHA) is allowed to rest against the wellbore wall, filtercake can continue to grow around the contact point, and must be yielded in order to free the BHA. Emphasis has therefore been placed on developing thin, highly impermeable but weak filtercakes, which minimise their contribution to severity of differential sticking, and also ease clean-up in the reservoir section.
More recently the focus on sealing fractures to prevent lost circulation of drilling fluids has turned interest towards strengthening and reinforcing filtercakes. As a result, several techniques have been developed to measure filtercake strength and related rheological properties. However, most of these rely on measurements on depressurised cakes, typically by extrusion or “hole punch” methods. These techniques have given robust data consistent with the applied filtration pressure, discriminated between different fluids, and shown the effect of various additives. However, the behaviour of the cake under pressure has remained intractable. The mudcake under pressure can have a higher yield stress, due to possible strain relaxation when the cake is depressurised.
Bailey et al (SPE 39429) disclose a method of measuring the strength of filtercake using a hole-punch or perforation technique. However, the measurement is performed on depressurised filtercakes and therefore does not simulate typical wellbore drilling conditions.
Wollny et al (Applied Rheology 11, 197-202 (2001)) discloses a technique for dewatering a filtercake between a filter and a measuring plate which maintains a zero applied load. A differential pressure is generated across the filter by a vacuum pump, but this method cannot achieve the high pressures typical of well-bore drilling fluids. Rheological properties of the filtercake are measured by oscillating a measuring plate across the filtercake surface.
Tomas and Reichmann (Chem. Eng. Technol. 25 1053(2002)) discloses an apparatus for the rheological testing of compressed filtercake based on the combination of a compression-permeability cell with a ring shear tester. A filtercake is formed on a filter by filtering a mud in a ring chamber. A ring piston is used to apply a compressive force and to measure the filtercake shear strength. However, this apparatus is not suitable for small samples and/or thin filtercakes.
UK Patent application GB 2275342 A discloses an apparatus and method for measuring the sticking tendency of drilling mud. The apparatus comprises a modified API-specified HPHT half-area filter press, containing a porous surface and a stainless steel ball adjacent to the porous surface. The filter press is filled with a mud to be tested and pressurised with nitrogen. The mud filters through the porous surface under pressure to form a filtercake on the porous surface, the filtrate leaving the filter press through a drain. The filtercake grows around a part of the ball which contacts the porous surface. The ball is then rotated about an axis normal to the porous surface and its resistance to rotation is measured to give an indication of the sticking tendency of the mud. In this device, there is no means to vary the thickness of the filtercake formed between the filter and the ball.